It could be dirt, but more than likely, it is an area where air was not completely exchanged for resin. You are sure you have done a good job...but when you remove the injector to add the pit filler and cure... there is that annoying area! When I am doing bullseyes, before the final finish, I go to vacuum, slide the injector off, and if there is any such spot, I slide it back into place, go to pressure, and cure under pressure before removing the injector. Yes, this does prolong the time of the repair, but the finished result is far better. Remember, too, that we tend to be our own worst critics. Most clients are very pleased with the results, since the visable damage is greatly reduced.